12 



Fishery Bulletin 100(1) 



swimming crabs on these arid beaches. Our study is thus 

 important both ecologically and as a preliminary assess- 

 ment of stock levels and their potential for sustainable ex- 

 ploitation. Also, few studies address the guild structure of 

 portunid crabs in the Caribbean Basin (Taissoun, 1969, 

 1973a; Moncada and Gomez 1980; Buchanan and Stoner 

 1988), in spite of their importance to fisheries. 



The purpose of our study was to widen our knowledge 

 of the population biology of A. cribrorii/s and various spe- 

 cies of Ca/lincctes in the Southern Caribbean. We quanti- 

 fied the distribution and population aspects of several spe- 

 cies of Callinectes and A. cribarii/s on a sandy beach and 

 in a small estuary located on an arid shoreline in Falcon, 

 western Venezuela. We present data on the distribution 

 and abundance of a guild of portunids, their physical cor- 

 relates, and some population characteristics of the domi- 

 nant species in marine and estuarine habitats. 



Materials and methods 



Sampling area 



Ensenada de La Vela (ir27'N. 69°34'W) is a small cove 

 located on the central coast of the State of Falcon in west- 

 ern Venezuela next to Istmo de Medanos, a narrow low- 

 lying isthmus that links Peninsula de Paraguana to the 

 mainland (Fig. 1). Ensenada de La Vela lies at the center 

 of a region that extends from eastern Panama to the Paria 

 Peninsula in eastern Venezuela that has been regarded as 

 abnormally dry (Lahey. 1973). In Falcon's western coastal 

 strip and in the Peninsula de Paraguana, rainfall is scarce 

 and seasonal with peaks during October-December, when 

 nearly 50-60''* of the precipitation occurs. The average 

 yearly rainfall on the peninsula is 400 to 600 mm (God- 

 dard and Picard, 1976), although in some localities the 

 environment is harsher with a mean of 256.8 mm (Conde 

 and Diaz, 1992b). The mean air temperature is 27.7°C, 

 and the average wind speed is 10 kph and seasonal (God- 

 dard and Picard, 1976). No mangroves, coral reefs, or sea- 

 grass beds appear at Ensenada de La Vela. 



Sampling procedures 



All samplings were carried out outside the fishing areas 

 used by local fishermen. Our study was conducted in three 

 stages. During the first and second stages, crabs were 

 sampled at two major biotopes. where three open-water 

 marine stations at the foreshore and one enclosed estua- 

 rine station were set. Station 1 was located on the east- 

 ern corner of Ensenada de La Vela, the most protected 

 area, where the surf action is extremely weak. Station 

 2 was also located in the marine front, but next to the 

 inlet of a natural estuarine impoundment where a nearby 

 basin drains. Station 3 was established in this estuarine 

 area, which is not freely connected to the ocean because 

 its exchange mouth is blocked by a sandbar for most of 

 the year. Station 4 was also set at the oceanic front, but 

 in a zone exposed to rugged surf next to an area that has 

 the strongest wave action in Venezuela. Marine stations 



were characterized by sandy bottoms, low-transparency 

 waters, and low-lying coastal profiles, whereas the estu- 

 arine station was mainly composed of muddy substrate, 

 and had little water movement. Most of the bottom was 

 devoid of vegetation and was interspersed with scarce flat 

 rocky patches. As a whole, some 300 m of shoreline were 

 trawled during each sampling. 



During the first stage of sampling, trawling and envi- 

 ronmental measurements were conducted monthly at each 

 station from January 1993 to December 1994. Field cam- 

 paigns included both rainy and dry seasons. Crabs were 

 collected using a 7-meter long and 1.5-m tall hand seine 

 (locally known as a "chinchorro"), with a mesh diameter of 

 1.0 cm. The hand seine was trawled perpendicular to the 

 coast by two persons walking alongside the net in a 20-m 

 long strip, parallel to the coast, at a depth ranging from 1 

 to 1.5 m. We repeated this process four times consecutively 

 at approximately the same site. This is the procedure that 

 local fishermen use to capture shrimp and fish to optimize 

 fishing effort. Because waves break and depth does not 

 exceed 1.5 m at our sampling sites, these sites were un- 

 suitable for the other types of fishing gear used in other 

 studies for capture of swimming crabs, such as crab pots, 

 trot lines, otter trawls, and dip nets, among others (Shol- 

 ar, 1979; van Montfrans et al., 1986; Prager et al., 1990; 

 Ryer et al., 1990; Hsueh et al., 1992). Our method was also 

 used by Carmona-Suarez and Conde (1996) to inventory 

 brachyuran crabs in the State of Falcon, Venezuela. After 

 each trawl at a given station, captured crabs were put in a 

 bucket until sampling at that specific site was completed. 

 When all four trawls were completed and the data were 

 recorded, crabs were returned alive to the same place. 



Because the behavior of some swimming crabs has been 

 hypothesized to follow daily cycles of burrowing in the sub- 

 stratum during the day and of emerging at night (Warner, 

 1977; Fischer, 1978), we conducted a second stage of sam- 

 plings from September 1997 to February 1998 at the same 

 sites. We sampled during the day and the night of the 

 same day or on consecutive days to determine if there 

 were any systematic differences in the composition of the 

 guild, in the abundance, size, and sex ratios of crabs, and 

 to detect potential biases introduced by the sample design 

 used in stage 1. The gear and sampling design for stage 

 2 was the same used during stage 1, but we were unable 

 to collect crabs at the estuarine station because of a dis- 

 charge of untreated domestic sewage at the beginning of 

 1997. 



The purpose of the third sampling stage was to assess 

 if guild composition and other characteristics were influ- 

 enced by depth or by distance from the coast. Three new 

 stations (inshore, midshore, and offshore) were sampled si- 

 multaneously in August 1998 and from June to July 1999, 

 from the foreshore oceanward at intei-vals of 50 m (Fig. 1). 

 The depth at the inshore station was -1.5 m, and the 

 depths at the mid- and offshore stations were -2.5-3 m. 

 During this stage, two crab pots with fish bait (as described 

 by Oesterhng, 1984) were deployed at each station for ap- 

 proximately 12 hours during the day, and for the same 

 amount of time during the night, except in August 1998, 

 when pots were deployed for 6 hours during the day and 



